The invention relates generally to voltage-frequency converters, and more particularly, to a voltage-frequency converter for use in the electric control system of motor vehicles.
In a known voltage-frequency converter, a differential amplifier is used as a comparator. The positive or non-inverting input of the comparator is connected to receive a constant reference signal and the comparator, negative or inverting, input is connected to the input terminal of the converter. The comparator output is connected to the D input of a D flip-flop. The Q output of the D flip-flop is connected via a resistor to the negative input of the comparator. A capacitor, which is used as a storage or memory device, is connected between the comparator negative input and ground. A frequency signal corresponding to an analog input signal supplied to the converter input terminal is present and available for use at the output of the D flip-flop.
The fundamental principle of this known voltage-frequency converter is the alternating charging and discharging of the capacitor, with the frequency of the change in charge being dependent on the input voltage; thus a voltage-frequency conversion is provided.
As a rule, the output signal of the D flip-flop is carried together with a clocking signal to an AND or a NAND gate at the output of which a frequency-controlled signal is then available for use.
The known circuit functions generally satisfactorily. However, it does have its weaknesses wherever the input signal of the comparator must be associated with an absolute, fixed potential as a reference voltage. Problems occur, because in practice the identical reference voltage is very frequently required within a relatively comprehensive system, but fluctuations occur and accordingly problems in adapting levels to one another because of the withdrawal of current from this reference voltage source which is then made. It is true that these problems can be surmounted with a high level of technological expense; however in terms of mass production, these complicated stabilization circuits prove to be too expensive.